Agriculture Reference
In-Depth Information
poorly available unless treated with alkali, such as soaking corn in lime water. The
need for niacin may be partially or totally met by hepatic synthesis of NAD from
tryptophan in some species if dietary tryptophan supplies are adequate (i.e., if tryp-
tophan supplies exceed needs for protein synthesis). Recommended dietary allow-
ances range from 6 mg per day for 1- to 3-year-old children to 18 mg per day for
pregnant women.
P a n t o t h e n i C a C i D
Pantothenic acid is a water-soluble, yellow, viscous oil that is relatively unstable
unless combined with calcium to form white, crystalline calcium pantothenate. It
functions in the body as a component of 4′-phosphopantetheine and CoA. Synthesis
of these compounds requires pantothenic acid, Mg 2+ , the amino acid cysteine, and
adenosine triphosphate (ATP). They are important in liberation of energy from car-
bohydrates, lipids, and proteins and in synthesis of heme (for hemoglobin), choles-
terol, steroid hormones, bile salts, fatty acids, and ketone bodies. CoA is involved in
acetylations of certain proteins, thus affecting their activity, location, and function.
CoA also acetylates amino sugars that function structurally to provide molecular
recognition sites on cell surfaces. Choline is acetylated by CoA to form the neu-
rotransmitter acetylcholine.
Much of pantothenic acid in food is present as CoA. During digestion, CoA is
hydrolyzed first to pantetheine and then to pantothenic acid. The latter appears to be
absorbed principally from the jejunum—at low concentrations by a sodium-depen-
dent, active process and at high concentrations by passive diffusion. Pantothenic
acid is then transported via plasma in free form to the tissues, where the majority is
converted to 4′-phosphopantetheine and CoA. This vitamin is widely distributed in
foods and is found in quite good concentrations in meats (particularly liver), eggs,
legumes, broccoli, potatoes, and whole-grain cereals. Vitamin supplements usually
contain calcium pantothenate or panthenol. Estimated adequate intakes range from 2
mg per day for 1- to 3-year-old children to 7 mg per day for lactating women.
v i t a m i in b 6
Vitamin B 6 occurs in three interchangeable forms, pyridoxine, pyridoxal, and pyri-
doxamine, each having a 5′-phosphate derivative. Pyridoxal phosphate (PLP) is the
principal coenzyme involved in amino acid metabolism (transamination, decarboxy-
lation, transulfhydration, desulfhydration, cleavage, racemization, and synthesis),
although pyridoxamine phosphate (PMP) also functions as a coenzyme in transami-
nations. In addition, PLP is involved in synthesis of heme, sphingolipids, niacin,
carnitine, and taurine and participates in glycogen catabolism and modulation of the
activity of some steroid hormones.
Pyridoxine and pyridoxine phosphate (PNP) are the forms found mostly in
plants, although pyridoxine β-glucoside is sometimes present. PLP and PMP are the
forms found primarily in animals. To be absorbed, phosphorylated forms must be
dephosphorylated by intestinal phosphatases, one of which (alkaline phosphatase) is
zinc dependent. Absorption of the free forms into the enterocyte occurs by passive
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